1. Field of Endeavor
The present disclosure relates to a fault current limiter, and more particularly to a fault current limiter used in electric power systems.
2. Background
A fault current limiter is a means of detecting a fault current generated by lightning, grounding and short-circuit and limiting the current to a normal level within several seconds, using a superconductor as a current limiting device which has substantially no resistance until certain up to a predetermined current value but rapidly represents a high resistance over a predetermined current value to limit the conducting current.
The fault current limiter is concentrated with a huge amount of energy due to resistance generated by the superconductor, such that energy consumption of the superconductor increases as the voltage applied to the superconductor increases.
That is, a superconductor exhibits zero resistance during its normal operation state in a system, however, when a fault current flows in the system, the superconductor is quenched to produce resistance which limits the fault current. At this time, the resistance may cause considerable energy to be applied to the current limiter. As a voltage applied to the system that runs the superconductor is high, the energy flowing in the superconductor correspondingly increases due to the impedance produced at the superconductor. Accordingly, lots of superconductors are needed to distribute the energy.
Thus, to minimize the energy consumption of the superconductor, a large number of the superconductors is needed which leads to increasing the manufacturing cost, and a total volume increases in accordance with use of huge number of superconductors, thereby increasing the installation and cooling cost.
That is, the superconductor is expensive in price and lots of superconductors mean large volume, which may increase the costs for installing and cooling the superconductor. To overcome the above problems, a hybrid-type superconducting fault current limiter (SFCL) including an existing circuit breaking means and a small number of superconductors has been disclosed. But the suggestion has failed to solve the price problem. As capacity of the resistive superconducting fault current limiter becomes large, size of the linear coils and number of windings have to be increased thereby to have a disadvantage in cost and operation.
FIG. 1 is a circuit diagram illustrating a structure of a hybrid-type superconducting fault current limiter according to prior art, FIG. 2a is a current graph of the hybrid-type superconducting fault current limiter of FIG. 1, FIG. 2b is a graph illustrating an operation start time of the hybrid-type superconducting fault current limiter of FIG. 1, FIG. 2c is a graph illustrating an arc current in the hybrid-type superconducting fault current limiter of FIG. 1, and FIG. 2d is a current graph of the hybrid-type superconducting fault current limiter where electric arc was not blocked by the main circuit of the fault current limiter.
Referring to FIGS. 1 and 2, an electric current Itot passes through the closed circuit breaker 210 and superconductor 100 (Imain) during a normal operation state without any fault, so that loss caused by occurrence of resistance is substantially ‘0’. However, in a case where a fault current (Ifuse) flows into the circuit, the superconductor 100 starts to be quenched (A) at a very high speed, and the fault current (Ifuse) bypasses to the driving coil 220 due to impedance developed at the superconductor 100.
At this time, a magnetic field is generated by the current flowing into the driving coil 220, and an eddy current having a diamagnetic component is induced at an electromagnetic repeller 230 located over the driving coil 220.
Accordingly, the repeller 230 moves fast and opens a circuit breaker 210 mechanically linked with the repeller 230 thereby to cut off the inflow of the fault current into the superconductor 100 (B).
However, in the fault current limiter thus configured, the minute that the circuit breaker 210 is open, an arc current occurs across the circuit breaker 210, which causes the fault current to continue flowing into the superconductor 100. To eliminate the arc current, the fault current limiter is designed to close a short contact 240 that is mechanically linked with the electromagnetic repeller 230 (C).
That is, the whole fault current is transferred through the short contacts 240 to an auxiliary circuit, and therefore, the arc current across the circuit breaker 210 is eliminated (D) and then the fault current is transferred to the auxiliary circuit and reduced by the current limiting unit 300 (E).
However, in the course of limiting the fault current thus explained, the electric arc occurring across the circuit breaker 210 that is connected in series with the superconductor 100 may not be sufficiently removed before the current limiting unit 300 starts to operate due to difference in impedance between a main circuit formed with the superconductor 100 and the high-speed switch and the current limiting unit 300-formed auxiliary circuit that functions to limit the current (F).
Accordingly, an electric arc (arc current) is reproduced across the circuit breaker 210 due to the difference in impedance between the main circuit and the auxiliary circuit (G), which can reduce the arc impedance, so that the fault current can go through the superconductor 100 that changes into a normal conductive state and the circuit breaker 210 that becomes conductive due to the electric arc. At this time, most of voltage is applied to the superconductor 100 that is in a normal conductive state, so that the fault energy may flow into the superconductor 100, thereby damaging the superconductor 100.
FIG. 3 is a circuit diagram illustrating a hybrid-type superconducting fault current limiter according to prior art, proposed to solve the aforementioned problems, where a power semiconductor element 400 is added thereto. The power semiconductor element 400 is added to the main circuit to block an arc current generated after the superconductor 100 is quenched.
However, the fault current limiter according to FIG. 3 also fails to solve the problems, because superconductors are employed and the number of superconductors must be adjusted to adjust the operating current to thereby increase the costs for installing and cooling the superconductor and to reduce convenience in manipulation.